This invention relates to a code division multiple access (CDMA) mobile communication system comprising a mobile station, a base station, a base station control apparatus, and an exchange station and, in particular, to a forward transmission power control method in the CDMA mobile communication system.
In recent years, as a communication system for use in a mobile communication system, attention is directed to a time division multiple access (CDMA) communication system which is not affected by interference and interruption. The CDMA communication system is a communication system in which a transmitting side spreads a user signal to be transmitted using a transmission spreading code to transmit a spread signal and a receiving side despreads the spread signal using a reception spreading code identical with the transmission spreading code to obtain an original user signal.
In addition, in the CDMA mobile communication system, a plurality of transmitting sides carry out spreading operations using different transmission spreading codes having orthogonality, respectively, and the receiving side can carry out identification of each communication by selecting, as the reception spreading code, one of the transmission spreading codes to be used on carrying out despreading operation. Accordingly, it is possible to use the same frequency band in a plurality of communications.
However, it is difficult to perfectly maintain the orthogonality among all of the spreading codes to be used. In actually, the respective spreading codes are not perfectly orthogonal to one another and then each spreading code has a correlation component with respect to other spreading codes. As a result, the correlation component serves as an interference component in its own communication and it results in a degradation factor in communication quality. Inasmuch as the interference component occurs due to such a degradation factor, the larger the number of communications becomes, the larger interference components becomes. As a result, it is possible to increase the number of communication channels enable to multiplex in a frequency band if each communication channel has a low transmission power.
As a result, each base station in the CDMA mobile communication system carries out control of transmission power for each mobile station so that each of received bit energy-to-interference ratios Eb/I0 (which are ratios of desired received wave powers (Eb) to interference wave powers (I0)) obtained by communication channels established between the base station and the respective mobile stations is equal to a necessary power ratio Eb/I0 required to insure the minimum communication quality. This transmission power control is called a reverse transmission power control.
Specifically, a transmission power control (TPC) information signal for instructing up/down of transmission power for the mobile station is included in a forward signal which is transmitted from the base station to the mobile station via a forward link. By using the TRC information signal, the base station carries out an up instruction or a down instruction of a current transmission power for each mobile station.
In the above-mentioned CDMA mobile communication system, each base station covers a cell which is a area where the base station can communicate with the mobile station. In addition, the mobile station moves with a communication channel established between the mobile station and a base station. When the mobile station goes out of the cell covered by the base station, the mobile station must carry out a handover for establishing a new communication channel with another base station which cover a cell where the mobile station moves.
The mobile communication system adopting a spread spectrum system carries out a soft handover in order to maintain a service quality by avoiding a momentary interruption or the like on carrying out the handover between the base stations. The soft handover is the handover where the mobile station simultaneously carries out communication with a plurality of base stations and is a peculiar function to the spread spectrum communication which can use the same radio frequency.
However, inasmuch as the same information signal is transmitted from the plurality of base stations to the mobile station when the soft handover is carried out, transmission power in the forward link per communication is much. As a result, interference power affecting other communications increases and it results in a restriction of a communication capacity.
In order to improve such a restriction, for instance, a method of decreasing transmission power in the forward link per communication by carrying out forward transmission from only one base station having a minimum propagation loss on the soft handover is disclosed in an article contributed by Furukawa (NEC Corporation) to the Communication Society Conf., IEICE, B-5-118, March 1998.
However, it is impossible for this conventional forward transmission power control method to secure a user quality in a case where there is a large fading in a communication channel. This is because the forward transmission is always carried out from only one base station on the soft handover.
In a multimedia communication, prediction is made that a transmission rate where a large capacity of data is transmitted from a data base at a network side to the mobile station is large. When the rate of the multimedia is large in future, it is necessary to increase a radio channel capacity of the forward link in comparison with that of the reverse link. As a result, it is necessary to decrease the transmission power of the forward link.
In addition, as regards the reverse link which is a communication channel from the mobile station to the base station, one transmission is always carried out per communication and an effect of the soft handover is obtained by receiving in a plurality of base stations. Accordingly, the problem of increasing the transmission power does not arise on carrying out the soft handover.
At any rate, the above-mentioned conventional CDMA mobile communication system is disadvantageous in that the communication capacity of the forward link is restricted on carrying out the soft handover. This is because the transmission power of the forward link per communication increases and interference power affecting other communications increases.
In addition, various other CDMA mobile communication systems related to the present invention are already known. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 8-172,390 or JP-A 9-172390 discloses a mobile communication system which is capable of reducing the influence of fading by selecting a plurality of base stations the reception powers from which at a mobile station exceed a prescribed value and simultaneously transmitting signals from these base stations to the mobile station with different frequencies and subjecting them to diversity synthesis. According to JP-A 8-172390, transmitted from a mobile station, a signal is received by first and second base stations, and a control station performs diversity synthesis. Transmitted from the first and the second base stations, signals are received by the mobile station and are subjected to diversity synthesis. When the mobile station moves to a point in the vicinity of the boundary of a first radio zone for the first base station and the reception power from the first base station is reduced to the prescribed value or smaller, communication with the first base station is switched to a third base station which the reception power from is larger. When the mobile station moves a point in a second radio zone for the second base station and the reception power from the second base station is reduced to the prescribed value or smaller, communication with the second base station is switched to that with a fourth base station which the reception power from is larger. In the similar manner, communication with the third base station is switched to that with a fifth base station at a point in the vicinity of the boundary of a third radio zone for the third base station.
Japanese Unexamined Patent Publication of Tokkai No. Hei 6-13,956 or JP-A 6-013956 discloses a transmission power controller in a mobile communication and its system which are capable of increasing the capacity of the system by implementing accurate transmission power control at a mobile station side. According to JP-A 6-013956, a control processor detects received power and the reception power is calculated from a strength of a pilot signal fed from a searcher receiver in this case. Thus, the signal strength of a transmission signal from a base station from which a mobile station is communicated is accurately detected. Furthermore, a fading speed is estimated from a change in the received power thereby controlling an averaging time in a transmission power controller. Thus, the system copes even with a change in the fading speed and the optimum averaging time is always obtained. Then an electric field median strength is always detected without causing a response delay and accurate transmission power control is attained.